The present invention pertains to a porous, sound- and/or heat-insulating shaped element, in particular for lining of sound- and/or heat-emitting vehicle components, comprising a bonded fiber fabric made of natural fibers mixed with a thermoplastic binder and shaped in a pressed mold and solidified, and a method for its production.
Shaped elements of this type are used preferably in the automobile industry where they are attached as insulating mats to the bottom of the chassis, to the front wall or in the engine compartment or trunk. For a tight fit these shaped elements are shaped according to the particular contours of the automobile in which they are to be used. Depending on their use, they can be provided with a cloth-like decor or a dirt-repellent protective coating.
In addition to the technical properties, such as acoustically active insulating ability and aesthetic aspects, today the health-related aspects of the shaped elements are moving into prominence. It is known that the release of substances from the shaped element produces not only unpleasant odors, but also can cause illness and allergic reactions. This applies specifically to bonded fiber fabric stiffened with phenolic resin.
In general these shaped elements are manufactured by hot pressing into suitable shaped bodies. In this case a so-called semifinished product--that is, a bonded fiber fabric of natural and/or synthetic fibers--is placed into a heatable mold with powdered binder, in particular phenolic resin, for example, Novolak, which polymerizes thermally at an appropriately high temperature, and is produced in the desired shape. The Novolak resin can be , but is not limited to, a phenol/formaldehyde or a urea/formaldehyde type resin.
A conventional method for the production of shaped elements from bonded fiber fabric is described e.g. in U.S. Pat. No. A 4,327,049. The formed fabric consists of a laminate of thermoplastic material, such as polystyrene or polyurethane foam and is placed into a heatable mold press, heated to soften it, and then pressed into shape. In this case it is difficult to achieve a homogeneous distribution of heat across the entire bonded fiber fabric and thus to obtain a shaped article with the same physical, in particular, acoustical insulating properties throughout--especially for shaped articles of more than 80 mm thickness. Moreover, at the elevated press temperatures, the risk of decomposition of the substances employed increases, and thus also the risk of release of undesirable decomposition products.
To avoid the disadvantage of poor heat transfer, in U.S. Pat No. 5,108,691 a High Caloric Transfer Medium (HCTM) process is proposed, where the bonded fiber fleece, e.g. resin-bonded glass fibers, is exposed to flowing, superheated steam, in addition to the heat treatment in the press mold. This steam penetrates through the formed fabric at a suitable temperature and thus allows an increase in shaping temperature in the interior of the mold.
However, it turns out that this method will work well only when using semifinished products with the same density at all points. Articles with differing densities however, will be exposed to the steam in differing degrees, so that it may happen that many sites will already be hardened, but others will still feature a high percentage of nonpolymerized material. Naturally this will again present the problem of release of substances hazardous to health, such as formaldehyde or ammonia.
An attempt has also been made (see U.S. Pat No. 4,623,499) to pass a reaction gas through the closed press mold to effect the catalytic polymerization reaction. It turns out however, that this flow technique likewise is not able to produce an odor-free shaped article. In particular, areas of good and poor flow seem to appear in the shaped article. Likewise, differently pressed regions or regions with elevated water content appear to lead to locally greatly differentiated reaction initiation times. At any rate, shaped elements with a thickness greater than 30 mm cannot be satisfactorily hardened with this method and they exhibit burn phenomena at the access points of the flowing medium.
Another significant disadvantage of the method described above is that powdered binder present in the semifinished product is carried off by the flowing medium and this leads to local inhomogeneities in binder distribution. Moreover, resins blown off in this manner usually condense at the outlet openings of the press mold and require time-consuming manual maintenance and cleaning. However, the undesirably large quantities of toxic substances (formaldehyde, ammonia, phenol, etc.) released by incomplete crosslinking of the binder have proven to be particularly troublesome. Evidently those regions where insufficient heating is produced are particularly critical, so that the resin probably has released ammonia and formaldehyde, but the resin itself is not able to crosslink with the formaldehyde due to insufficient temperature levels.
In addition, the regions with insufficiently crosslinked binder are subject to rapid decomposition, and as a secondary effect to the unpleasant odor mentioned above, they will form potential foci for rot and mold.
Therefore it is an object of the present invention to create a shaped element which does not have the disadvantages of the known shaped elements and in particular, which features a low emission potential for toxic substances and unpleasant odors.
Possible approaches to achieving this object may be to extend the process time and/or to increase the process temperature. In this regard it must be remembered that the maximum permissible temperature is at around 180.degree. C. to prevent carbonizing of the natural fibers. Therefore the percentage of binder has to be chosen so that a sufficient stability will be achieved in the poorly crosslinked regions of the shaped element. The relatively high percentage of noncrosslinked binders or cleavage products is to be taken into account here to ensure the necessary strength.